Nitrogen dynamics in a small arctic watershed: retention and downhill movement of15N

Abstract

We examined short‐ and long‐term nitrogen (N) dynamics and availability along an arctic hillslope in Alaska, USA, using a stable isotope of nitrogen (15N), as a tracer. Tracer levels of15NH4+were sprayed once onto the tundra at six sites in four tundra types: heath (crest), tussock with high and low water flux (mid‐ and footslope), and wet sedge (riparian).15N in vegetation and soil was monitored to estimate retention and loss over a 3‐year period.Nearly all15NH4+was immediately retained in the surface moss–detritus–plant layer, and >57% of the15N added remained in this layer at the end of the second year. Organic soil was the second largest15N sink. By the end of the third growing season, the moss–detritus–plant layer and organic soil combined retained ≥87% of the15N added except at the Midslope site with high water flux, where recovery declined to 68%. At all sites, non‐extractable and non‐labile‐N pools were the principal sinks for added15N in the organic soil.Hydrology played an important role in downslope movement of dissolved15N. Crest and Midslope with high‐water‐flux sites were most susceptible to15N losses via leaching, perhaps because of deep permeable mineral soil (crest) and high water flow (Midslope with high water flux). Late spring melt season also resulted in downslope dissolved‐15N losses, perhaps because of an asynchrony between N release into melt water and soil immobilization capacity. We conclude that separation of the rooting zone from the strong sink for incoming N in the moss–detritus–plant layer, rapid incorporation of new N into relatively recalcitrant‐soil‐N pools within the rooting zone, and leaching loss from the upper hillslope would all contribute to the strong N‐limitation of this ecosystem. An extended snow‐free season and deeper depth of thaw under warmer climate may significantly alter current N dynamics in this arctic ecosystem.